This invention relates to radiography and more particularly to an improved scanning X-ray system and method for producing image data indicative to internal characteristics of subjects to be examined.
X-ray derived images of living subjects or mechanical structures are most commonly obtained by using an X-ray source in which a small area anode or target is bombarded with accelerated electrons. The resultant X-rays diverge from what may be regarded as essentially a point source. The divergent X-ray beam, usually consisting of a continuous wave-length spectrum, passes through the subject and impinges on a photographic film which must then be developed to produce a visually perceptible image. In other instances, the divergent X-ray beam is received on a broad screen formed in part of a material which fluoresces visible light in response to X-radiation.
There are several characteristics which detract from the utility of X-ray systems of the form discussed above and which tend to limit the quality of the images which are obtained. Most notably, a sizable radiation dosage is required to produce a clear image and this constitutes a potential hazard to operator personnel as well as to an invivo subject. A large proportion of the X-rays which are generated and passed through the subject do not produce a discernible actinic or fluorescent reaction in the film or screen and thus are essentially unutilized. Where photographic film is used, a delay is necessary while the film is being developed. Further, the required equipment tends to be undesirably bulky and heavy and consumes a substantial amount of electrical power. If stereoscopic images are desired, these several undesirable characteristics, including high radiation dosage, are aggravated since much of the equipment must be duplicated for this purpose.
Except in certain special circumstances such as in dental radiology, it is usually a practical necessity to produce an image which includes the full depth of a subject. Because of the necessarily large size of the photographic film or the like, it is often not practical to insert such means into a human body or into convoluted recesses within mechanical structures which are to be examined. However, in many instances more useful data could be obtained by imaging only a portion of the total depth of a subject. Further, the commonly employed X-ray techniques are not susceptible to localized contrast control in different areas of the image except by cumbersome techniques, such as localized partial masking, which are not fully effective and which have undesirable side effects.
Another serious problem in the above-described conventional X-ray system results from the largely unavoidable production of secondary X-rays within the subject being viewed. In order to achieve maximum definition and clarity in the image, all X-rays which react with the film or screen must have a single small area of origin at the source. This is not usually achieved in practice since a certain proportion of the primary X-rays from the source interact with individual atoms within the subject in a manner which causes secondary X-rays to be emitted from within the subject. As these secondary X-rays do not originate at the same point as the primary X-rays, interaction of the secondary X-rays with the film or fluorescent screen degrades the quality of the image. The fact that the film must often be disposed as close to the subject as possible aggravates this problem as this means a large proportion of the secondary X-rays impinge upon the film.
It is a common practice to reduce degradation from secondary X-rays by disposing a focussing grid between the subject and the film or screen. Such a grid is formed of radiation opaque material and has a series of radiation transmissive passages aligned to transmit only X-rays travelling along paths which converge at the X-ray source. The grid may be oscillated to avoid superimposing an image of the grid itself into the image of the subject. Such grids are only partially effective for the desired purpose, may themselves be a source of secondary X-rays, and have the undesirable effect of still further increasing the amount of X-radiation which must be generated to produce an acceptable image.
It is often desired to store records of X-ray examinations, particularly in medical and dental applications, and the physical size, fragility, and cost of X-ray film creates complications in this connection. Further, where X-ray film is used as a data storage medium, complex equipment and operations are required if it should be desired to process the data by electronic image enhancement techniques.
In order to avoid certain of the problems discussed above, it has heretofore been proposed to provide a moving point source of X-rays whereby the subject may be systematically scanned in a raster pattern. The X-radiation passing through the subject may then be detected electronically and displayed on a television or cathode ray tube which has a raster pattern and sweep frequencies coordinated with that of the source. Prior U.S. Pat. No. 2,730,566 describes one such systems. A basic advantage of such a system is that electronic radiation detectors can be more sensitive than photographic film or fluorescent screens and therefore the required radiation dosage of the subject may be reduced.
As heretofore designed, such scanning systems do not fully resolve certain of the problems discussed above and have no significant advantage over the more commonly used film technique insofar as others of these problems are concerned. These prior X-ray scanning systems necessarily require a very large area radiation detector which, in effect, replaces the photographic film or fluorescent screen discussed above.
Suitable radiation detectors of the necessary size are not manufactured in quantity by commercial suppliers, and at best are inherently costly and subject to reduced sensitivity and an increased spurious cound factor. Further, a large detector cannot usually be inserted into a living body or into restricted regions within apparatus to be examined.
To produce any useful data at all, the prior X-ray scanning systems necessarily require that a broad multi-apertured focussing collimator or grid be positioned between the scanning X-ray source and the radiation detector so that the only X-rays counted at any specific instant are those which have passed through one specific region of the subject. This tends to reduce image definition as X-radiation is alternately absorbed within the collimator and then transmitted therethrough many times in the course of each scan, in effect reducing the image to a mosaic of dots.
Although some secondary X-rays may be absorbed in the collimator, many pass through the openings thereof and are counted by the detector as in the more conventional X-ray film process.
For these and other reasons, prior X-ray scanning systems have not replaced the photographic film technique, at least to any great extent, notwithstanding the reduction in radiation dosage which can be inherent in such systems.